An ultrasound diagnostic imaging system, which can display two-dimensional image (such as B-mode imaging, color flow imaging etc.) and Doppler spectrogram in real time, has an important sense for clinical diagnosis. Whereby, a clinical doctor can obtain the status of blood flow in a certain Doppler sampling volume while observing anatomical structures in real time. There are two ways to obtain the status of blood flow, i.e. a real-time spectrogram displayed on a monitor and a real-time Doppler audio signal played from a speaker. In order to realize the above-mentioned real-time displays under two (even three) different imaging modes, the ultrasound imaging system assigns different time segments to different imaging modes using time division multiplexing technique and the system switches quickly among different modes periodically. Taking the B+D mode (i.e. B mode plus pulsed wave spectral Doppler mode) as an example, which is used commonly in clinic, the system alternates scan between two modes by controlling a time sequence of transmitted pulses. The simplest way is to transmit a single B pulse and a single Doppler pulse alternately. The main shortcoming of this way is reduction of the Doppler pulse repetition frequency (PRF), which causes significant decrease in a detectable maximum flow velocity. Another similar way is to transmit a single B pulse and a plurality of Doppler pulses alternately. Although this way has improved the maximum flow velocity detected by Doppler, the time for imaging a single frame of B-type image is prolonged, thus unfavorably influencing the real-time display of 2D image. Considering the imaging quality of 2D image, the present invention concerns a different kind of scanning, in which the imaging system initiates transmission of Doppler pulse after scanning over a frame of 2D image and the number of the transmitted Doppler pulses is large enough to obtain at least one Doppler power spectrum. Since a Doppler signal in 2-40 ms is usually regarded as a quasi-stationary signal, the gap length is required to be less than 40 ms as much as possible. Because the Doppler signal can not be acquired during time periods for B-mode imaging, it causes discontinuity of the Doppler signal. The discontinuous phenomenon (namely “Gap”) exhibits a broken spectrogram, as shown in FIG. 1, which may lead to a failure of automatic Doppler spectral parameters estimation, and even introduce periodical abrupt mute in audio output, thus degrading the audio output to loss the reference significance for clinical diagnosis. Therefore, the technique for filling Doppler signal gaps has become an important one necessary for an ultrasound diagnostic imaging equipment.
The technique for filling Doppler signal gaps is used to improve the quality of spectral Doppler signal in an ultrasound diagnostic imaging equipment, especially useful when the ultrasound diagnostic imaging equipment is working in both 2D imaging mode (such as B-mode, color flow imaging mode etc.) and spectral Doppler mode. The ultrasound diagnostic imaging system realizes simultaneous imaging under different modes using time division multiplexing technique, and the system assigns different time segments to different imaging modes. When the system is working in both the 2D imaging mode and the spectral Doppler mode, there will be an evidently discontinuous phenomenon (namely “Gap”) occurring in audio Doppler signal and spectrogram, which will bring great inconvenience for clinical diagnosis. In addition, the imaging frame rate of 2D image is decreased, although there is a relatively small influence on diagnostic performance. The Doppler signal gap filling technique fills the gaps by means of signal processing, so that the filled signal maintains a better continuity both in audio and in spectrogram, thus reducing the influence of the discontinuity of the acquired Doppler signal on clinical diagnosis. However, how to fill Doppler signal gaps effectively has always been a challenge in applications of spectral Doppler technique. With development in digital computer technique, researchers have proposed a lot of methods to solve this problem. According to output way of the spectrogram, the current methods for filling gaps are generally divided into two main classes: one is to fill 2D spectrogram directly by interpolation, and another is to calculate the spectrogram of the gap-filled signal.
A filling method is disclosed in U.S. Pat. No. 5,016,641 issued on May, 1991 to G. Schwartz, entitled “Spectral Interpolation of Ultrasound Doppler Signal”, according to which the power spectrum before a gap is used to fill the power spectrum during the gap, thus producing a phase with certain randomness; a time-domain signal can be obtained by inverse fast Fourier transform (IFFT) according to the filled power spectrum with random phases; and a filled signal with no step can be obtained after the time-domain signal is weighted by means of a smooth window function and partially superposed. This method needs to execute IFFT each time the spectral line is updated, increasing the computation complexity.
A method for filling gaps in spectrogram and audio separately is disclosed in U.S. Pat. No. 5,476,097 issued on December, 1995 to M. T. Robinson, entitled “Simultaneous Ultrasonic Imaging and Doppler Display System”, in which signals before and after a gap are filled in the gap in an inverse sequencing to guarantee the continuity at ends of the gap, but there is a jump in the joint. The noise caused by the discontinuity can be eliminated by digital-to-analog conversion followed by low pass filtering. However, because the spectrum of the signal is inverted after inverse sequencing of the quadrature Doppler signal, there is an inverse error in audio output after flow direction separation. According to this U.S. patent application Ser. No. 5,476,097, the spectrogram is filled from the middle of the gap to both sides respectively by means of the spectra at both ends of the spectrogram gap, and each spectrum is filled more than once (for example twice) until the filled spectrum is equal to the original spectrum in the filled time. The discontinuous spectra among them can be smoothed by interpolation or averaging.
A method for utilizing an artificial neural network to estimate spectra during a gap is disclosed in the paper of H. Klebak, J. A. Jensen and L. K. Hansen, “Neural Network for Sonogram Gap Filling”, published in Proceedings of IEEE International Ultrasonics Symposium, vol. 2: 1553-1556. However, the computation complexity of this method is too large to meet the real-time demand for the ultrasound diagnostic imaging system.
When the gap is short enough, the signals near the gap have similar statistical characteristics according to the quasi-stationary characteristic of the Doppler signal so that the Doppler signal in the gap may be estimated according to the signals before and after the gap. A method for utilizing a signal before a gap to estimate a signal in the gap by linear prediction is disclosed in the paper of K. Kristoffersen and B. A. J. Angelsen, “A Time-shared Ultrasound Doppler Measurement and 2-D Imaging System”, published in IEEE Trans. Biomed. Eng., vol. 35: 285-295. The linear prediction is a method that estimates the Doppler signal in the gap by means of the least mean square error.
A method for filling Doppler signal gaps based on model estimation is disclosed in U.S. Pat. No. 5,642,732 issued on January, 1997 to J. S. Wang, entitled “Apparatus and Method for Estimating Missing Doppler Signals and Spectra”. In this method, first, a variance of exciting noise and AR model coefficients of the Doppler signal before the gap are estimated, and reflection coefficients of the model are estimated from these AR coefficients. Similarly, noise variance, reflection coefficients and AR coefficients of the Doppler signal after the gap are estimated. Then, the reflection coefficient during the gap is interpolated (Interpolating AR coefficient directly may cause instability of the system, because the necessary and sufficient condition of the system stability is that amplitude of the reflection coefficient is less than 1, while the Burg estimation method can guarantee that the amplitude of the reflection coefficient is less than 1). Finally, AR coefficients are calculated from the estimated reflection coefficients, according which an IIR filter is constructed. The IIR filter is excited by white noise, and the filtered data is windowed and then is superposed and jointed with the actually acquired signal. Then, the spectra of the gap-filled Doppler signal can be computed by using the fast Fourier transform. In addition, since the power spectrum of the signal can be estimated directly from AR coefficients through the fast Fourier transform, the estimated power spectrum can also be directly output by normalizing the variance of the interpolated noise. This method can obtain continuous voice and spectrogram outputs. However, when the bandwidth of the signal is relatively wide, it is necessary to approach the spectrum by means of AR model of a higher order. Especially when there are both forward and reverse flow signals in the detected signals of blood flow, it may introduce much estimated error of AR model parameter, thus degrading the quality of the filled spectrogram and audio unfavorably. Additionally, the estimation by AR model of a high order has greater computation complexity, increasing the cost to implementation.
Two methods for producing a gap-filled signal are disclosed in U.S. Pat. No. 4,559,952 issued on December, 1985 to B. A. J. Angelsen and K. Kristoffersen, entitled “Method of Ultrasonically Measuring Blood Flow Velocity”: 1. A broadband noise is passed through a filter to produce a signal to be filled, in which the coefficient of the filter can be controlled to produce the required spectral characteristics; 2. A Doppler signal in the last stored portion is read directly. In order to guarantee the continuity, the produced signal and the acquired signal are weighted by a window function and there may be an overlap portion between them. The method is applicable to the quasi-stationary Doppler signal. Nevertheless, large arterial blood flow may cause the flow velocity to increase or decrease fast during cardiac systole. In this case, discontinuity of spectrogram is produced easily when the gap length is relatively long.
A detailed method for utilizing broadband noise through a filter to produce a filled signal is disclosed in U.S. Pat. No. 4,934,373 issued on July, 1990 to B. A. J. Angelsen and K. Kristoffersen, entitled “Method and Apparatus for Synthesizing a Continuous Estimate Signal Provided by Ultrasonic Doppler Measurement on a Fluid Flow”. The main idea is that the acquired signal is directly windowed and then used as coefficients of a FIR filter, and the same power spectrum with random phases can be obtained after the broadband noise is passed through the filter. While implementing the method, considering that change in bandwidth of a signal is relatively slow, the signal is demodulated to the baseband frequency using an average frequency and the demodulated signal is windowed as the coefficient of the filter. After estimating the average frequency in the filled time, the filtered baseband signal will be modulated to a higher frequency by means of the estimated average frequency, obtaining the signal in the filled point. In order to guarantee the continuity of the estimated signal, the estimated signal is weighted by a window function and then superposed before output. According to this method, the original acquired Doppler signal and the Doppler signal in the gap are replaced with the result filtered by the filter directly, addressing the problem of the discontinuity of the audio and spectrogram. However, nonstationarity of the Doppler signal and window weighting of the filter coefficients may produce an over-estimated spectral bandwidth.
A method for utilizing Doppler data before and after a gap to fill the gap is disclosed in U.S. Patent Application 2007/0049823 filed on March, 2007 by Y. Li, entitled “Method for Processing Doppler Signal Gaps”, in which the data before and after the gap are first passed through a high pass filter, and then read in a positive sequence and filled in the gap. Front section of the gap is filled with the data before the gap and its rear section is filled with the data after the gap. For the discontinuous section at the joint, window weighting is used to converge the data to zero to guarantee data continuity. However, this method can not address the problem of the discontinuity of spectrogram occurring when the flow velocity is fast increased or decreased. Moreover, as a result of converging the data to zero by window weighting of the signal at the joint, the signal energy will change periodically, and the spectrogram will exhibit a periodic bright-dark change, while audio will exhibit a periodic strong-weak change.
A method for utilizing Doppler data before a gap (or before and after a gap) to smoothly fill the data in the gap is disclosed in U.S. Pat. No. 5,891,036 issued on April, 1999 to M. Lzumi, entitled “Ultrasonic Wave Doppler Diagnosing Apparatus”. In the first embodiment, the data before the gap is read in a reverse sequence and then is stored in the gap position after conjugation (or exchanging real part with imaginary part). Since the start point of the gap can produce a discontinuous phase after conjugation, a phase deviation at this point is then calculated to compensate for the data in the gap. This method can realize a totally smooth joint at the start point of the gap, whereas there may be discontinuity at the end point of the gap. In the second embodiment, the same process is carried out on the data after the gap in consideration of the data before the gap. Thereafter, the processed data before and after gap are weighted and then superposed to guarantee continuous, smooth transitions in both ends of the gap. This method can not either address the problem of the discontinuity of spectrogram occurring at the time of fast increasing or decreasing of the flow velocity.
The present invention provides a new method for filling Doppler signal gaps, which utilizes the Doppler signal before and/or after the gap to fill it. The method is different from the prior art in that frequency modulation is performed on the Doppler signals before and/or after the gap before that signal and the acquired Doppler signals are weighted by a window function and superposed, thus obtaining continuous spectrogram and audio outputs. This method maintains an excellent original spectral characteristics of Doppler signals.